In the related art, known examples of systems that detect the air pressure of for example the tires of an automobile include a tire air pressure monitoring system (Tire Pressure Monitoring System: TPMS). In general, a TPMS is attached to the inside of a tire and includes a sensor that detects the air pressure and temperature of the tire and a transmitter that wirelessly transmits data to a transceiver arranged in the vehicle. Accordingly, in a TPMS, it is necessary that there be a power source for supplying power to the sensor and the transmitter, for example.
In the related art, a variety of power generators have been proposed as power sources that can be used in TPMS's and the like. Among these generators, piezoelectric power generators that utilize the piezoelectric effect have particularly been the focus of attention, since this kind of generator can be easily reduced in size.
For example, the piezoelectric power generator illustrated in FIG. 18 is described in below-cited Patent Document 1. As illustrated in FIG. 18, a piezoelectric power generator 100 includes an elastic body 102. One end portion of the elastic body 102 is fixed in place and a weight 101 is attached to the other end portion of the elastic body 102. Contacts 103 and 104 are provided on main surfaces 102a and 102b of the elastic body 102. In addition, piezoelectric elements 105 and 106 are fixed above the main surfaces 102a and 102b. 
In the piezoelectric power generator 100, when the elastic body 102 is displaced in the upward direction due to application of a stress to the weight 101, the contact 103 applies a pressure to the piezoelectric element 105. As a result, the piezoelectric element 105 is deformed and power is generated in the piezoelectric element 105. In addition, when the elastic body 102 is displaced in the downward direction due to a stress being applied to the weight 101, the contact 104 applies a pressure to the piezoelectric element 106. As a result, the piezoelectric element 106 is deformed and power is generated in the piezoelectric element 106.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-312269